1. Field of the Invention
The present invention concerns a magnetic resonance imaging (MRI) system having a housing that allows the system to be used in environments that must not be exposed to the high magnetic field of the system.
2. Description of the Prior Art
Mobile magnetic resonance imaging systems have been known for some time. An MRI system represents a substantial financial investment, which may be beyond the means of many communities and local health authorities. In order to provide MRI services to such communities, some businesses provide mobile MRI systems which can be made available in a certain locality for a certain time before moving on to a next locality. Each community or local health authority will only pay for the time that the mobile MRI system is at their disposal. A mobile MRI system typically occupies the trailer of an articulated HGV, and travels on the public highways. By careful planning and scheduling of imaging procedures, MRI imaging is made available to communities and local health authorities which would otherwise be denied such facilities, or would have to arrange for patients to travel, sometimes significant distances, to attend a fixed MRI centre.
Legislation, for example in the United States and the European Union, places restrictions on the intensity of magnetic field which may be emitted into the surrounding environment by installations such as mobile MRI trailers (known as the stray magnetic field). Presently, the maximum allowed stray magnetic field intensity is 5 gauss. In the context of trailer-mounted mobile MRI systems, regulations require the 5 gauss magnetic field intensity contour to be within the external surfaces of the trailer up to a height of 8 feet (2.5 m) from the ground on which the trailer is sitting.
Present mobile MRI systems typically employ superconducting magnets having a magnetic field strength of 1.5 T or less. The magnetic fields of such magnets can be constrained within the body of the trailer of an HGV, by such expedients as active shield coils within the magnet, and ferromagnetic shielding of the trailer walls, as discussed for example in United States Patent Application Publication No. 2006/0186884 A1.
It is generally accepted that improved quality images can be achieved in MRI if a magnet of increased magnetic field strength is used. Current fixed MRI systems routinely employ magnets having a magnetic field strength of 3 T. It is desired to use such magnets in mobile MRI facilities. However, it has been found difficult to constrain the stray magnetic field of magnets having a magnetic field strength much in excess of 1.5 T to within the dimensions of an HGV trailer. For example, FIG. 1 illustrates a model of stray magnetic field emitted by a current 3 T magnet placed within a mobile MRI system trailer designed to accommodate a 1.5 T magnet. The trailer in question is a Medical Coaches “AVANTO” United States-specification trailer. The outermost magnetic field contour line shown in FIG. 1 represents the 5 gauss magnetic field strength contour. As can be seen, the 5 gauss contour extends significantly beyond the walls of the trailer, despite the trailer being sufficient to constrain the magnetic field of a 1.5 T magnet such that its 5 gauss contour falls within the walls of the trailer. In the example shown in FIG. 1, the trailer walls are unable to constrain even the 50 gauss contour.
One solution may be to add further passive shielding to the trailer, in the form of iron plates on the walls of the trailer. However, it has been found that the additional mass provided by the necessary quantity of iron shielding would make the resultant trailer impractically heavy for road transport. Furthermore, it is known that even small displacements of shielding material, due to thermal expansion or the effect of wind for example, will interfere with the stability of the magnetic field. This is already an issue for screened trailers for 1.5 T magnets, and would become a more serious issue if further screening were introduced.
Another alternative would be to increase the dimensions of the trailer such that the 5 gauss contour remains within the walls of the trailer. This solution has also been found impractical, as the resultant size of trailer required to transport a 3 T magnet would be too large for normal road transport.
Further conceivable alternatives include providing additional shield coils within the magnet to reduce the stray magnetic field at source. These shield coils could be superconducting coils in the cryostat or resistive coils mounted externally to the cryostat. Both options are expensive, add weight, and in the latter case would require significant power and cooling to operate.